Electrical communication circuit



May 23, 1933. s. BALLANTINE ELECTRICAL COMMUNICATION CIRCUIT Original Filed Jan. 9 1923 Reissued May 23, 1933 3- NITED STATE-S PATENT OFFICE,

sa'uncar BALLAN'LINE, or noun'rnnv LAxEs, NE JERSEY, assrenon, BY 1:15am! nssremmv'rs; 'ro RADIO CORPORATION or AMERICA, or NEW YORK, mm, A con- PORATION OF DELAWARE EL EG. 1RICAL COMMUNICATION CIRCUIT Original lie. 1,728,719, .dated August 6, 1929, Serial Ho. 611,647, filed January 9, 1923. Application for reissue filed June 13, 1930. Serial No. 461,027.

This invention relates to electrical communicatlon circuits, and particularly to C11- cuits which may include one or more ampliof the characteristic, and distortion always fying audion tubes.

It is well known that the audion tubes which are employed in telephone repeating circuits and in radio-telephone transmission and receiving circuits do not give an exact enlarged reproduction of'the wave impressed upon the grid of the audion. The distortion arises from the fact that the plate currentgrid voltage characteristic of audiontubes-is not linear, but is more or less curved throughout its entire extent. When an audion is employed as an amplifier, the distortion produced is comparatively small when the operating point is located on the linear part.

of the aforementioned characteristic curve, but some distortion is always present. When used as a detector, however, the operating point of the audion is on the curved portion results, sincethe amplitude of the rectified current varies approximately as the square of the amplitude of the impressed voltage. This invention has for an object the provision of an electrical communication circuit which may include one or more amplifying audions in which the distortion produced by the audion or audions is reduced or eliminated, such reduction or elimination of the distortion being often referred to as neutralizing the distortion. A further object is to provide a circuit of the type stated, in which the amplification ratio of one or more of the audions is varied to produce a correction which will reduce or eliminate the distortion normally present in such circuit. More specifically an object of the invention is'to provide a' radio receiving. circuit including an audion amplifier and a detector, in which'the amplifying ratio of the audion is controlled to introduce a correction which will reduce or eliminate the distortion produced by the detector.

The accompanying drawing illustrates an secondary of the transformer 6 is connected through the tunedcircuit 1 to the grid. of the amplifying audion 2. The connection between the low side of the secondary winding of the transformer 6 and the amplifying audion filament includes a C battery 7 hav ing appropriatemeans including a potentiometer 7 for varying the biasing voltage impressed upon the grid of the audion. The

telephones 8, preferably shunted by a fixed condenser 9, are connected between the B battery 10 of the amplifier audion and the negative side of the A battery 11. The plate circuit of the detector includes the usual B battery 12 between the audio frequency transformer and the detector filament. Condensers 13, 14 across the audio transformer primary and B battery'12 and across the audio transformer secondary, respectively,-

provide means for by-passing radio frequency oscillations around the transformer 6. .It

' will be observed that the circuits established provide for the simultaneous amplification of radio and audio frequency oscillations in the y audion 2, but this feature is-not essential to my invention, as the telephones may beincluded in the plate circuit of the detector.

The arrangement as illustrated is preferable, however, since it permits the use of an audio transformer of the usual construction, and the full audio frequency drop in the plate circuit occurs across the transformer.

In operation, the voltage of the C battery is adjusted to bias the amplifying audion to bring the operating'point upon a curved portion of the plate current-grid voltage characteristic. When a modulated radio frequency wave is received, the current is amplified by the audion 2, passed through the vario-transformer 3 to the detector 5, where it is rectified. 'Theresulting audiocurrents pass into the audio-transformer ,6 and causes a second audio frequency bias voltage to be impressed upon the amplifying audion 2 in phase with, or eiractly opposite in phase to,

the modulated radio frequency voltage which thls audion is passing through to the detector. Since the detector operates according to the square law, i. e., the detected audio frequency is proportional, roughly, to' the squareof the amplitude of the envelop of the vradio frequency wave, the peaks inthis envelo are unduly extended in the audio wave,

pro ucing an alteration in the quality of the audio tone. Due to the fact that the constant bias from the C battery has brought the operating point to a curved portion of the aforementionedtharacteristic, the audio bias produces a variation in the amplifying ratio which causes the audion 2 to pass a modulated radio frequency wave in which the peaks are flattened. This flattening of The correcting bias may be introduced in the peaks of the envelop of the radio frequency wave produces an envelop having such variations from the original wave shape as to reduce or eliminate the distortion produced by the detector.

ft will be obvious that the invention'is not limited to a radio receiving circuit of the type shown since a correcting bias voltage of substantially the samewave form as an" audio frequency wave or the modulated envelop of a radio frequency wave may be applied to an audion of other amplifying circuits for the purpose of reducing or eliminating the distortion normally present in such a circuit.

telephone repeating circuits, in radiophone transmitting circuits, or in audio frequency amplifying circuits, and the bias may be impressed upon one or more of the audions of the circuit. In radio telephone reception,

the invention is not limited to circuits em-' ploying an audion detector, but is useful with any detector .WhiCh produces a distorted audio wave, i. e., one which does not have the same form as the envelop of the.radio' frequency wave.

Iclaim; I 1. In an electrical communication circuit,

. an elementproducing a distortion in the shape of the wave traversing said circuit, an amplifying audion preceding said element, and means cou ling the element and amplifying audion or varying the amplifying.

ratio of said audion to introduce a correction neutralizing the distortion introduced by "said element.

pr ss n i p 2. Ina radio receiver comprising 9. cascaded radio frequency am lifier of' the audion ty e and a detector 0E ducing in erent distortion tions between said audionand detector for the input circuit of said amplifier afidion audio frequency response inherent property nal ener thetype procircuit .cOnnecenergy of said detector, and means impressing a negative bias potential upon said audion of a magnitude suflicient to bring the operating point of said audion to a curved portion of its plate current-grid voltage characteristic.

3. Method of operating an electrical circuit including a vacuum tube amplifier and a detector to neutralize distortion inherent in the'detector which comprises adjusting said amplifier to operate upon a curved portion of its plate current-grid voltage characteristic curve, detecting the amplified currents by the detector having the inherent distortionand employing said detected currents to control the amplifying ratioof said amplifier in such a manner as to produce therein a neutralizing distortion. I

4. Method of operating an electrical circuit including a vacuum tube amplifier and a detector to neutralize distortion inherent inthe detector which comprises biasing said amplifier to operate upon a curved portion of its plate current-grid voltage characteristic, impressing alternating currents upon said amplifier input, detecting the amplified currents ,by said detector having inherent distortion, and employing said detected currents to vary the bias ofsaid amplifier in such a manner as Y to produce a neutralizing distortion.

-5. Method of operating a vacuum tube detector' to neutralize inherent distortion there-' to that of the inherent distortion of said transmission element, impressing upon said element the distorted signal-wave produced by said amplifier, and impressing upon a reproducer the signal wave of corrected form that is produced by said transmission element.

7. The method of automatically controlling the output of an electrical communication system of the type including an audion amplifier effective to amplify a received sig-. nal of the modulated carrier wave type and a detector for-demodulating the amplified sigto produce an audio frequency current, w 1ch method comprises impressing upon the grid of saidaudion amplifier a di rect current bias of such magnitude that said amplifier. operates u on a-curved portion of its plate current-grljd voltage characteristic curve, and superposing upon said applied di rect current bias a second potential developed by said detector and varying in magnitude with rectified signal energy, thereby automatically to vary the amplification of said audion amplifier in accordance with the magnitude of received signal energy.

8. The method of automatically controlling the outputof a radio receiver of the type including a radio frequency amplifier and a detector, which comprises impressing the re ceived signal energy upon the radio" frequency amplifier, impressing upon an input electrode of said audion amplifier a direct current potential of such magnitude that said amplifier operates upon a curved portion of its plate current-grid voltage characteristic, passing the amplified energy to said detector, and transferring to said amplifier electrode as an amplification-controlling bias apotential derived from said detector-and varying in magnit'udewith rectified signal energy developed by said detector.

9. The invention set fOIth in claim wherein said direct current potential plification-controlling potential are impressed upon the control grid of said amplifier audion.

. 10. In an electrical communication system,

the combination with a high frequency am plifier. of side band modulated carrier energy and means impressing upon an-amplificatinncontrol circuit of said amplifier a direct current potential appropriate for the normal namplification of relatively weak carrier energy, of means efi'ective automaticaly to re duce the amplification of said amplifier as the carrier strength increases, said second means COIIIPIlSlIlg a rectifier, means transand am- 12. In a radio broadcast receiver, a radio frequency amplifier provided with a tuned input circuit adapted to have carrier and signal modulation side band energy impressed thereon, a detector having its input circuit coupled to the amplifier output circuit whereby the amplified carrier and modulation side band energy is impressed on the detector, means in the cathode circuit ofsaid amplifier for normally maintaining the control electrode of said amplifier at anegmitting to said rectifier the modulated carrier output of said amplifier, and circuit elements for superposing upon said direct current potential a second potential derived from said rectifier and varying inmagnitude witli the rectified modulated carrier energy. I

11. In a radio broadcast receiver, a radio frequency amplifier ,provided with a tuned input circuit adapted to have carrier and signal modulation side band energy impressed thereon, a rectifier having its input circuit coupled to the. amplifier output circuit whereby the amplified carrier and modulation side band energy is impressed on the rectifier, means for normally maintaining the control electrode of said amplifier at a negative potential with respect to the cathode, and means for applying, between said amplifier control electrode and cathode, a voltage derived from the rectifieroutput circuit in a sense to oppose variations in the signal energy in the rectifier output circuit, which last named variations are produced by variations in intensity of the signal energy impressed on' said amplifier tuned input circuit. 

